The present invention relates to a method of producing conductor circuit boards, and more particularly, to a method of producing conductor circuit boards which is capable of restraining the excessive growth of an edge bead at the boundary between a conductor circuit and a resist mask.
To produce conductor circuit boards, there is a conventional method in which a thin metal film is formed over the entire surface of an electrically conductive substrate which is planar in shape, for example, a resist mask for obtaining a desired circuit pattern is formed on the thin metal film, and a conductor circuit pattern is formed on the surface of the thin metal film except the portions thereof where the resist mask pattern is formed, by an electrolytic plating method using the thin metal film side of the planar conductor substrate as a cathode. Then, an insulating substrate is pressure-bonded to the circuit pattern-side surface of the planar conductive substrate with heat applied thereto, to obtain a laminate, and only the planar conductive substrate is separated from the laminate, thus transferring the conductor circuit pattern to the insulating material.
Thus, the electrolytic plating method is employed to form a conductor circuit pattern on the thin metal film. As such plating method, the so-called high-speed plating method capable of forming a high-quality conductor circuit at high speed is proposed in U.S. Pat. No. 4,790,902. In this method, as illustrated in FIG. 30, first, a thin metal film 2 is formed over the entire surface of a planar, electrically conductive substrate 1, and a resist mask 3, 3a is formed on the surface of the thin metal film 2 except the portions thereof on which a conductor circuit pattern is to be formed. Subsequently, the planar conductive substrate 1 is positioned such that the surface thereof on which the thin metal film 2 is formed is spaced at a predetermined distance from a planar anode (not shown), and a plating solution is supplied at high speed between the anode and the metal film-side surface of the conductive substrate 1 serving as a cathode, thereby depositing copper on the conductor circuit-forming portions of the cathode surface to form conductor circuits 4 and 4' with a desired thickness.
According to this method, however, if some conductor circuits of the conductor circuit pattern to be formed, for example, the conductor circuits 4 and 4', are located far from each other, that is, if the conductor circuits 4 and 4' are individually isolated in position from the other circuits, so-called edge beads (also called "dog-bones") 5 and 5' are concentratively formed at the boundary between the conductor circuit and the resist mask 3a corresponding to the blank portion between the conductor circuits 4 and 4', as shown in FIG. 30. The edge beads 5 and 5' are produced as a result of the phenomenon that an electrolytic current concentratively flows through the edge portions of conductor circuits that are in the process of being formed during the electrolytic plating and accordingly a plating metal is excessively deposited on the edge portions.
If this phenomenon occurs, the thickness and width of the conductor circuits become much greater than their respective design values. As a result, air may be trapped near the edge beads, or in some cases, the edge beads may penetrate into the insulating substrate when the conductor circuits are transferred to the insulating substrate, to cause an accident such as short-circuiting. Further, when the conductor circuit board is to be used as a high-frequency circuit board, the widths of the conductor circuits and the insulation spacings between the circuits must be strictly set in order to ensure satisfactory characteristics. In such cases, if the edge beads are produced during the formation of the conductor circuits, the high-frequency characteristics of the substrate are extremely deteriorated.
The growth of the edge beads 5 and 5' is promoted with increase of the current density during the electrolytic plating. For example, there are occasions when the ratio of the height t' of the edge beads (5, 5') to the thickness t of the middle portion of the conductor circuits (4, 4'), i.e., t'/t, is 1.9 at the least or as large as 5 to 6 (FIG. 30).
To solve this problem, a method of restraining the growth of edge beads has been proposed in which, before carrying out the electrolytic plating step, a dummy circuit is formed between the conductor circuits 4 and 4' which are isolated from each other (see Japanese Provisional Patent Publication No. 58-123793).
According to this method, if the dummy circuit is not to be incorporated into the final conductor circuit board, only the dummy circuit must be removed before the conductor circuit pattern on the planar conductive substrate is transferred to the insulating substrate.
It is, however, very difficult to remove only the dummy circuit, since the thin metal film 2 is formed over the entire surface of the planar conductive substrate 1.
If no thin metal film is formed and the dummy circuit and conductor circuits having respective predetermined patterns are formed directly on the surface of the planar conductive substrate, the dummy circuit could be removed relatively easily. In this case, however, if the dummy circuit is removed, part of the surface of the planar conductive substrate is exposed between the conductor circuits. As a result, an insulating material, or a matrix resin of the insulating substrate, enters small pits present in the surface of the planer conductive substrate during the transfer step and firmly adheres to the conductive substrate, thus posing the problem that the conductor circuit pattern cannot be transferred.
Further, with the above-described method in which no thin metal film is formed on the entire surface of the planar conductive substrate 1, it is not possible to produce a conductor circuit board whose conductor circuits to be formed are electrically isolated from one another and in which a through hole must be formed in the isolated conductor circuits and through-hole plating is carried out. The reason is that the isolated circuits are not electrically connected and accordingly the inner surface of the through hole cannot be plated.